1. Field of the Disclosure
The technology of the disclosure relates to determining propagation delay in distributed radio frequency (RF) communications signals communicated over optical fiber in distributed antenna systems.
2. Technical Background
Wireless communication is rapidly growing, with ever-increasing demands for high-speed mobile data communication. As an example, so-called “wireless fidelity” or “WiFi” systems and wireless local area networks (WLANs) are being deployed in many different types of areas (e.g., coffee shops, airports, libraries, etc.). Distributed antenna systems communicate with wireless devices called “clients” (e.g., for WiFi services) or “user equipment” (e.g., for cellular services) which must reside within the wireless range or “cell coverage area” in order to communicate with an access point device.
One approach to deploying a distributed antenna system involves the use of radio frequency (RF) antenna coverage areas, also referred to as “antenna coverage areas.” Antenna coverage areas can have a radius in the range from a few meters up to twenty meters as an example. Combining a number of access point devices creates an array of antenna coverage areas. Because the antenna coverage areas each cover small areas, there are typically only a few users (clients) per antenna coverage area. This allows for minimizing the amount of RF bandwidth shared among the wireless system users. It may be desirable to provide antenna coverage areas in a building or other facility to provide distributed antenna system access to clients within the building or facility. However, it may be desirable to employ optical fiber to distribute communications signals. Benefits of optical fiber include increased bandwidth.
One type of distributed antenna system for creating antenna coverage areas, called “Radio-over-Fiber” or “RoF,” utilizes RF signals sent over optical fibers. Such systems can include head-end equipment optically coupled to a plurality of remote antenna units that each provides antenna coverage areas. The remote antenna units can each include RF transceivers coupled to an antenna to transmit RF signals wirelessly, wherein the remote antenna units are coupled to the head-end equipment via optical fiber links. The RF transceivers in the remote antenna units are transparent to the RF signals. The remote antenna units convert incoming optical RF signals from an optical fiber downlink to electrical RF signals via optical-to-electrical (O/E) converters, which are then passed to the RF transceiver. The RF transceiver converts the electrical RF signals to electromagnetic signals via antennas coupled to the RF transceiver provided in the remote antenna units. The antennas also receive electromagnetic signals (i.e., electromagnetic radiation) from clients in the antenna coverage area and convert them to electrical RF signals (i.e., electrical RF signals in wire). The remote antenna units then convert the electrical RF signals to optical RF signals via electrical-to-optical (E/O) converters. The optical RF signals are then sent over an optical fiber uplink to the head-end equipment.
It may be desired to provide such optical fiber-based distributed communications systems indoors, such as inside a building or other facility, to provide indoor wireless communication for clients. Otherwise, wireless reception may be poor or not possible for wireless communication clients located inside the building. In this regard, the remote antenna units can be distributed throughout locations inside a building to extend wireless communication coverage throughout the building. However, other services may be affected. For example, cellular communications protocols may be based on time based protocols. However, distributing cellular communications to remote antenna units inside a building or other environment can increase delay or introduce path dependent variability of observed delays due to the propagation delay of the communications signals being distributed to the remote antenna units and responses received at the remote antenna units being distributed back to head-end equipment. If the delay(s) exceeds a certain level, the bandwidth of the communications system may be reduced. Other undesired effects can also result from delay exceeding certain levels, including interference. As another example, providing localization services for a client, such as emergency 911 (E911) services, may be determined or calculated based on over-the-air delay between communications from a client and a communications tower. However, if the client is communicating over a distributed antenna system, the propagation delay of the distributed antenna system increases the delay thus possibly resulting in an incorrect determination of a location of the client.
Delay can be compensated for in a cellular communications system, but the cellular communications system may be unaware of indoor distributed antenna systems that increase the delay as a result of propagation delay. Also, a cellular communications system may provide for the ability to set a flag or other indicator to indicate that distributed antenna systems are present in coverage areas for communications antennas or towers. However, this setting may only provide for the ability of the cellular communications system to approximate additional propagation delay present which may not be accurate.